Apparatus for reducing aerodynamic drag in system for air cooling a high power vacuum tube

ABSTRACT

A high power vacuum tube 10 has a cylindrical body 16 and a plurality of radially extending fins 18. A cylindrical wall 20 is co-axially disposed about the body and the fins, thus defining an air channel between the body and the wall. To cool the tube, air is forced into the channel from an entry port, and leaves the channel through an exit port. The cooling capacity of the system is optimized by contouring the entry and exit ports to minimize aerodynamic drag. In one embodiment, the entry into the channel is flared outward and a cone assembly 42 is provided adjacent the longitudinal end of the body 16 nearest the exit port. This cone assembly thus forms a continuation of the body 16 which tapers to a smaller cross section in the direction of air flow. The cone assembly may be formed of a stack of discs 70 of successively smaller diameter. The exit port, which may have either rigid or flexible sidewalls, has a venturi shape to further reduce drag.

This is a continuation, of application Ser. No. 022,945 filed Mar. 22,1979, now abandoned.

BACKGROUND AND FIELD OF THE INVENTION

The present invention relates generally to the art of air cooling ofhigh power vacuum tubes, and more particularly to a system for reducingaerodynamic drag in the path of air flow through the system so as toincrease the cooling capacity of the system.

High power broadcasting stations, such as AM, FM, and TV broadcastingstations, conventionally employ large, high power vacuum tubes in theoutput stage of the transmitter for amplifying the modulated RF signalwhich is to be transmitted. These vacuum tubes conventionally operate atan extremely high power levels, measured in tens of kilowatts. Since thetubes are, of course, not 100% efficient, a portion of this high poweris converted into heat in the tube. Some means must therefore beprovided to dissipate this unwanted heat.

These high power vacuum tubes generally have a cylindrical bodyconfiguration, with a large number of heat radiating fins extendingradially outward from the body. These fins, which all have approximatelythe same length, are joined at their outer extremities by a wall, whichthus has a generally cylindrical configuration and is coaxially disposedabout the axis of the vacuum tube body. An air channel is therebydefined between the body of the vacuum tube and the cylindrical walljoining the extremities of the heat radiating fins.

High power vacuum tubes of this type are generally air cooled by forcinglarge volumes of air through this air channel. The size of the fan usedto generate this air flow must, of course, be selected in accordancewith the cooling requirements of the vacuum tube being used. For thosetransmitters using very high power output stages, cooling fans having avery high capacity must be employed. Effective cooling of the tube iscritical, since the useful life of the tube is adversely affected byoperation at elevated temperatures. It would therefore be desireable toprovide greater cooling of the tube so that it operates at a lowertemperature and thus has a longer useful life.

SUMMARY OF THE INVENTION

It has been found that this goal may be achieved, without increasing thesize of the fan employed, by reducing aerodynamic drag in the path overwhich the air must flow.

It is therefore an object of the present invention to provide a systemwherein aerodynamic drag in the air cooling path is reduced.

It is an additional object of the present invention to modify thecontour of the entry port of the air channel used for cooling the tubeso as to reduce the aerodynamic drag thereof.

It is yet another object of the present invention to modify the contourof the exit port of the vacuum tube air channel so as to reduce theaerodynamic drag thereof, as well.

It is still another object of the present invention to provide an exitport having flexible walls so that the air pressure within the exit portautomatically draws the exit port into the desired low dragconfiguration.

By modifying the contour of the entry and exit ports of the air flowsystem, it has been found that aerodynamic drag can be reduced by asmuch as 60%. This leads to a commensurate increase in cooling capacity,permitting the tube to operate at cooler temperatures than wouldotherwise have been the case.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the present inventionwill become more readily apparent from the following detaileddescription, as taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a sectional elevation view of the path of cooling air flowpast a high power vacuum tube, in accordance with the prior art;

FIG. 2 is a plan view of a conventional high power vacuum tube;

FIG. 3 is a sectional elevation view of the air flow path of FIG. 1,modified in accordance with the teachings of the present invention;

FIG. 4 is an elevation view, partly in section, of an alternativeconfiguration of an exit port in accordance with the present invention;

FIG. 5 is a sectional elevation view of another embodiment of an entryport in accordance with the present invention; and

FIG. 6 is an elevation view, partly in section, of another embodiment ofthe cone assembly used to reduce drag in the exit port.

DETAILED DESCRIPTION

There is illustrated in FIG. 1 a sectional view of a conventional highpower radio transmitter output stage. In this figure, a high powervacuum tube 10 is mounted at its lower end within an RF resonent cavity12, and at its upper end to a cylindrical chimney 14.

The high power vacuum tube 10 consists essentially of a generallycylindrical body 16 having a plurality of substantially rectangular heatradiating fins 18 extending radially therefrom. As can best be seen inFIG. 2, which is a plan view of the high power vacuum tube 10, byitself, these heat radiating fins extend from the inner body 10 outwardto a wall 20 having a generally cylindrical configuration, and coaxiallydisposed about the cylindrical body of the vacuum tube 10. An air flowchannel is thus defined between the body of vacuum tube 10 and the wall20. The air may therefore pass in a generally longitudinal directionpast the body 16 and the fins 18 to dissipate the heat developed by thetube.

The vacuum tube 10 illustrated generally in FIGS. 1 and 2 includes anumber of electrical contacts protruding from the portions of the body16 above and below the heat radiating fins 18. More particularly, aprotrusion 22 extends from the upper portion of the vacuum tube body 16,and provides one means for establishing electrical contact with theanode of the high power vacuum tube 10. The radiating fins 18 are alsoelectrically connected to the anode structure; electrical contact to theanode may therefore be established by simply clamping an electrical leadto the wall 20 joining the outer perimeter of the fins 18. In FIG. 1, anannular metal ring 23 is provided for this purpose, and has a pluralityof spring finger contacts 24 attached to the interior of the circularopening therein. These spring finger contacts resiliently engage thewall 20, establishing electrical continuity between the anode of thetube and the annular ring. An insulating sheet 27 of mylar isolates theannular ring from the top of the cavity, which will be grounded.

The electrical connections to the cathode, screen, grid, etc. of thetube 10 are not illustrated in the various figures, since they form nopart of the present invention. Conventionally, the box 25 upon which thevacuum tube 10 rests will be connected to the screen of the tube, withthe remaining electrical connections being made within this box.

In FIG. 1, the resonent cavity 12 within which the tube 10 is mounted iscomposed of a cover 26, sidewalls 28, and a floor 30, all constructed ofelectrically conductive material. The floor 30 of the cavity has ahoneycomb construction to permit air to pass through it substantiallywithout restriction. The floor 30 will normally be connected to a geardrive operable to move the floor 30 up and down along the screen box 24to permit tuning of the cavity 12. Air will be forced into the resonentcavity 12 by means of a conventional fan (not shown), and then will flowpast the heat radiating fins 18, and out through the chimney 14 (asindicated by the arrows 31).

In a system of this type, cooling of the high power vacuum tube 10 isdependent upon the flow of air past the heat radiating fins 18. A veryhigh rate of flow past these radiating fins is required for thosesystems operating at high power levels. Unfortunately, the very highaerodynamic drag normally associated with the air path shown in FIG. 1significantly restricts this air flow, hampering effective cooling ofthe high power vacuum. This has been found to be primarily the result ofthree factors: entry losses associated with the entry of the air flowinto the cooling fins from the resonent cavity; shear losses within theheat radiating fin structure itself; and exit losses associated with theexit of the air from the fin structure into the chimney. Although theshear losses are fixed by the structure of the tube and cannot be variedwithout altering the mechanical design of the fin structure, the entryand exit losses can be substantially reduced by modifying the contour ofthe entry and exit ports of the fin structure. This can lead to as muchas a 40%-60% reduction in the aerodynamic drag within the system, thusproducing a commensurate increase in the cooling capacity of the system.

FIG. 3 illustrates the system of FIG. 1, modified in accordance with thepresent invention. To simplify the description which follows,corresponding parts of the various figures are identified bycorresponding reference numerals.

In order to reduce entry losses, the embodiment illustrated in FIG. 3has been modified so that the entry port to the air cooling channel isflared outward. This is accomplished by means of a quarter-round annularring 32 having an outside diameter which closely matches the interiordiameter of the circular opening in the cover plate 26 through which thetube 10 extends. The quarter-round ring, which may be formed of Teflonor any other convenient insulating material, nests within this circularopening in the cover 26, and may be fastened therein in any convenientmanner. The quarter-round surface of the ring 32 faces the interior ofthe ring, and provides an aerodynamically smooth transition between thecavity 12 and the air cooling channel of the tube.

In some high power vacuum tubes, such as the one illustrated in thefigures, an additional anode connection 36 is provided circumferentiallyprojecting from the body 16 of the vacuum tube 10, somewhat below theheat radiating fins 18. This anode connection 36 has been troublesome inthe past due not only to the turbulence in the entry flow introduced byit, but also due to the tendency of dirt to collect in the dead airspace immediately above the connection. The collection of dust at thispoint led to the creation of "hot spots" in that vicinity, and hence topremature failure of the tube.

To eliminate both of these difficulties, it is contemplated that thisanode connection 36 will be covered with a contoured molding 38. Thismolding 38, which again may be constructed of any conventional heatresistent insulating material, will preferably be split at two pointsalong its circumference so as to present two half rings which can beeasily snapped into place over the anode connection 36. The actualcontour of the molding is not critical; the half-round cross sectionalshape illustrated has been found to function satisfactorily.

The inclusion of these two components, the quarter-round ring 32 and thecontoured molding 38, lead to a reduction in the aerodynamic dragassociated with the entry into the heat radiating fins down to a smallpercentage of its former value.

The exit losses associated with the system are substantially reduced byconnecting a cone assembly 42 to the top surface 44 of the body portion16 of the vacuum tube 10, and by providing an interior annular sidemolding 46 for modifying the interior contour of the chimney 14 in thevicinity of the exit from the air channel in which the heat radiatingfins 18 are located.

Although the cone assembly 42 may be entirely formed of a singlematerial, in FIG. 3 it is illustrated as being comprised of a topportion 48 constructed of an insulating material such as Teflon, and abottom portion 50 constructed of a highly heat conductive material, suchas aluminum. The purpose of providing this heat conductive portion 50immediately adjacent to top surface 44 of the vacuum tube 10 is toincrease the effective cooling area of the tube by conducting heat fromthe top 44 of the vacuum tube 10 through the aluminum portion 50 to thepath of air flow. Bottom portion 50 has an opening 52 therein forreceiving the anode connector 22 which protrudes from the top surface 44of the vacuum tube 10.

In FIG. 3, the top and bottom portions 48 and 50 of the cone assembly 42are fastened to one another by means of a machine screw 54 which isthreadedly received by the top portion 48 through an opening 56 in thebottom portion 50. The cone assembly 42 is then attached to the topsurface 44 by means of two machine screws 58 which pass through countersunk openings in the cone 42, and are received by tapped openings 60 inthe top surface of the vacuum tube 10.

For convenience of illustration, the cone assembly 42 is shown in FIG. 3as having a height equal to approximately twice its diameter. In actualuse, however, it is presently preferred that the cone have a muchgreater height, for example, equal to five times its diameter.

The interior annular side molding 46 is provided to modify the interiorcontour of the chimney 14 into a venturi shape, further reducing drag atthe exit from the heat radiating fins 18. This interior annular sidemolding will have generally the same contour as illustrated in FIG. 1,however its optimum contour will be determined by the particular systemin which it is employed, since it is affected not only by the particulartube 10 employed, but also by the air path treatment at the entry to theheat radiating fins 18. Again, this interior annular side molding may befabricated of Teflon, or of any other metallic or nonmetallic material.

Referring now to FIG. 4, a second method of imparting the desiredventuri shape to the chimney 14 is illustrated. In this embodiment, therigid sidewalls of the chimney 14 are replaced by a resilientcylindrical sidewall 60 (formed, e.g., of plasticized canvas, siliconrubber, etc.) connected at its lower end to the vacuum tube 10 by meansof a clamp 62, and at its upper end to a rigid diffuser 66 by means of asecond clamp 64. Although, when relaxed, this flexible sidewall 60 willhave a fixed diameter at all places therealong, when the system is inoperation the air pressures acting upon its interior and exteriorsurfaces will deform it into the desired venturi shape. Thus, when airis being forced through the heat radiating fins 18, the air pressure atthe exit port of the air channel housing the heat radiating fins 18 willbe somewhat reduced from the air pressure in the outside environment.The flexible sidewall 60 will thus be drawn inward at this point,automatically providing the desired exit contour. This embodiment isadvantageous not only in that the flexible sidewall 60 willautomatically be drawn into the desired venturi shape, but also becausethis venturi shape has a smooth, unbroken contour cover the entirelength of the chimney 14; this would be difficult to achieve in a rigidembodiment.

As is illustrated in this figure, it is further preferred that thechimney 14 include a diffuser 66 so as to reduce the velocity of the airflow at the exit from the chimney, thereby further reducing exit lossesthrough a mechanism known in the art as regain. This figure alsoillustrates the preferred height of the cone assembly 42.

Referring now to FIG. 5, an additional manner of modifying the contourat the entry to the heat radiating fins 18 is shown. This embodiment maybe preferred in those instances in which little additional space isavailable immediately below the heat radiating fins, or when the coolingair flow enters the cavity 12 from the side, and must be deflected upinto the air channel. In this embodiment, the quarter-round annular ring32 is replaced by a different circumferential ring 68 having ahalf-round cross section. This half-round circumferential ring may beheld in place in any convenient manner as, for example, by usingconventional set screws. It has been found that the inclusion of acontoured ring of this type, although not reducing drag as effectivelyas the bell shaped opening 32 described previously, nonethelesssignificantly reduces entry losses, leading to improved coolingperformance.

Referring now to FIG. 6, there is illustrated a second, and presentlypreferred, embodiment of the cone assembly 42. In this embodiment, thesolid cone is replaced by a series of spaced discs 70. A central stem 72passes through openings in the center of each disc, holding them in theproper axial alignment, while a series of cylindrical spacers 74 carriedon the stem establish the appropriate spacing between the discs. Thestem, spacer, and disc assembly is held together by means of two nuts 76which are threaded onto the top and bottom of the stem 72.

The disc spacing must be selected so that the assembly provides theappearance of a solid cone to the air flow. To accomplish this, it ispresently preferred that each disc be separated from the next smallerdisc by a distance approximately equal to one and one-half times thediameter of that next smaller disc.

In this embodiment, the cone assembly is not mounted directly onto thetube, but is rather held in proper alignment above the tube by twospiders 78. Each spider is clamped at its center 80 to the stem andspacer portion of the cone assembly 42, and includes four arms 82equally spaced circumferentially about the stem clamp and extendingradially therefrom to the side wall 14 of the chimney, indicated indotted lines in this figure. If a rigid sidewall is used the arms maysimply be bolted to the sidewall. If a flexible side wall is used, thearms will extend through slots in the flexible sidewall and be bolted toa frame member (not shown) outside of the chimney. In either case, thecone assembly will be held rigidly in place with respect to the vacuumtube 10.

The arms of the spiders will preferably have a very small crosssectional dimension in the direction perpendicular to the air flow, toreduce aerodynamic drag. Even more preferable, the spiders will have asmooth air foil shape to further reduce air drag.

Although the invention has been described with respect to preferredembodiments, it will be appreciated that the described embodiments areexemplary only, and are in no way intended to limit the scope of thepresent invention. Thus, the specific contours illustrated and describedfor the entries and exits to the air channel which houses the heatradiating fin structure may take many forms, with the preferred form ina specific application depending upon the actual cavity, tube andchimney parameters of that system. Also, of course, these streamliningfeatures may be integrated into the design of the tube, itself, ordivided between the system and the tube. Consequently, it will beappreciated that various rearrangements and alterations of parts may bemade without departing from the spirit and scope of the presentinvention, as defined in the appended claims.

What is claimed is:
 1. Apparatus for use in conjunction with a highpower vacuum tube including a body portion having a longitudinal axis, aplurality of heat radiating members each extending transversely outwardfrom said body portion in a direction transverse to said longitudinalaxis, channel means for defining a channel through which gas may flow ina longitudinal direction past said body portion and said heat radiatingmembers so as to cool said body portion, and entry and exit ports fordirecting gas flow into and out of said channel, said apparatuscomprising drag reducing means associated with at least one of saidentry and exit ports for providing an aerodynamically smooth transitionbetween said channel and said at least one port to minimize gas dragassociated with the flow of gas between said port and said channel,thereby improving gas flow through said channel, and improving thedissipation of heat from said high power vacuum tube, wherein said dragreducing means comprising chimney means associated with said exit portfrom said channel and defining an extension of said gas channel, whereinsaid chimney means is constructed of a material having sufficientresilience that reduced gas pressure created within the extension airchannel by the flow of gas therethrough will draw the chimney means intothe desired shape.
 2. Apparatus as set forth in claim 1, wherein saidchimney means has an opening for exhausting gas therefrom, and wheresaid opening is flared.
 3. Apparatus for gas cooling a high power vacuumtube including a body portion generating unwanted heat which must bedissipated by the flow of gas therepast, said apparatus comprising meansdefining a gas channel having an entrance for admitting gas into saidgas channel and an exit for exhausting gas from said gas channel, withat least a portion of the body of said vacuum tube being mounted withinsaid gas channel between said entrance and said exit, wherein a portionof said channel defining means downstream of the position along saidchannel at which said body of said vacuum tube is mounted has a reducedcross section and aerodynamically smooth transitions to and from saidreduced cross section so as to provide a smooth aerodynamic transitionbetween the portion of the channel containing said body of said vacuumtube and the portion downstream thereof, and wherein at least saidportion of said gas channel defining means downstream of the position atwhich said body of said vacuum tube is mounted is formed of resilientmaterial such that gas pressure within said gas channel draws thatportion of said gas channel into the desired shape.
 4. Apparatus for gascooling a high power vacuum tube including a body portion generatingunwanted heat which must be dissipated by the flow of gas therepast,said apparatus comprising means defining an gas channel having anentrance for admitting gas into said gas channel and an exit forexhausting gas from said gas channel, with at least a portion of thebody of said vacuum tube being mounted within said gas channel betweensaid entrance and said exit, and wherein at least a portion of said gaschannel defining means is constructed of a resilient material wherebythe shape of said gas channel in the vicinity of said at least a portionthereof will be determined by gas pressure within the gas channel atthat point.
 5. Apparatus as set forth in claim 4, and further comprisingmeans for providing said body portion with a shape which effectivelytapers along a line parallel to the direction of gas flow so as toreduce aerodynamic drag associated with the presence of said bodyportion within said gas channel.
 6. Apparatus as set forth in claim 5,wherein at least a portion of said means for providing said body portionwith a tapered shape is constructed of a highly heat conductive materialso that heat generated in said body portion is conducted to the taperedsurface of said means.
 7. Apparatus as set forth in claim 5, whereinsaid means for providing said body portion with a tapered shapecomprises a plurality of plates disposed in spaced relation to oneanother and said body portion along said line parallel to the directionof gas flow with the outer edges of said plates defining saideffectively tapered shape.
 8. Apparatus as set forth in claim 7, whereinsaid plates are oriented substantially parallel to one another andperpendicular to said line parallel to the direction of gas flow, saidplates being arranged in order of size, and wherein the spacing betweeneach plate and the next is dependent upon the size of that plate. 9.Apparatus for use in conjunction with a high power vacuum tube includinga generally cylindrical body, a plurality of heat radiating finsextending radially outward from said body, and a wall extendingcircumferentially around said fins whereby a channel exists between saidbody and said walls through which gas may flow to cool said body andfins, said apparatus comprising a chimney adapted to adjoin one end ofsaid channel for receiving gas from said channel, said chimney having asubstantially constant cross sectional size and shape matching the crosssectional size and shape of said wall, but having a venturi-likeconstriction near the axial end adjoining said channel whereby gas flowbetween said channel and said chimney is improved due to reducedturbulence in said gas flow.
 10. Apparatus as set forth in claim 9, andfurther comprising cone-shaped means disposed at the axial end of saidbody nearest said chimney for further reducing turbulence in said gasflow by providing said body with a cross sectional diameter which tapersin a regular manner in the direction of gas flow.
 11. Apparatus as setforth in claim 10, wherein said cone-shaped means comprises pluralspaced apart disks of differing diameters arranged in order of size. 12.Apparatus as set forth in claim 9 and further comprising means forproviding said circumferential wall with a flared entrance. 13.Apparatus as set forth in claim 9 and further comprising means forproviding said chimney with a flared exit.